Improving the T-962 Reflow Oven – Part 2

Owners of the T-962 reflow oven would agreed with me that the noise coming from that internal system fan is rather annoying. Unlike the big cooling fan at the back, which only comes on during the cooling down phase. That system fan starts to scream the moment it’s turned on. This certainly won’t go well in a quite office environment.

There are many ways to control the speed of a fan, but the preferred way to control the speed of the fan is to feed it with a pulse width modulated(PWM) signal, preferably in accordance with the temperature of the controller board. Compare with other methods, PWM has the advantage of low component counts and low heat dissipation, as the driving transistor is either fully on or fully off and not working in its linear region. After a few short discussions with Mr. Werner Johansson, the man behind the excellent firmware enhancement for the T-962 reflow oven controller, he quickly came up with the proof of concept patch, using the easily accessible free pin, ADO, on the controller board to PWM the system fan via a n-channel FET transistor. The driving circuit of this type can’t get any simpler and I just happened to have this simple circuit available on the clone board I made earlier. Q1 you see near the bottom of the board is the BSS138 n-channel FET. Click on the pictures to zoom in.

The flywheel diode is soldered to the underside of the board. Werner’s patch outputs the PWM signal to the ADO pin from the original controller. The next picture shows where the ADO pad is on the controller.

With Werner‘s new patch compiled and running on my controller, the system fan turns on briefly at startup. Unless the board’s temperature reach 40ºC, the system fan stays silent. As the board’s temperature increases, the speed of the fan will increase accordingly, At 50ºC, the fan will be running at it’s maximum. You can get Werner’s new patch here. (Oops it’s here). This is a highly recommended modification for all owner of T-926 reflow oven, as the noise from the system fan can really drive you nuts.

Below is the full schematic of my thermocouple amplifier as discussed in part 1. The system fan’s driving circuit is also included.

Apart from the two MAX31855 thermocouple amplifiers and the I2C-to-SPI bridge, the board also included a space for the DS18B20, the One-Wire temperature sensor, where it will now be used to measure the board’s temperature. The power will be taken from existing system fan’s socket, it’s around 10VDC. There is a socket for the existing system fan. Power to the fan is now PWM controlled via a n-channel logic-level Enhancement mode FET. Below is the picture of what the final board should look like.

I have no doubt with the performance of the MAX318555 as it has been used by many hobbyist to build their home-made reflow ovens. Nevertheless, I have seen reports regarding stability issues. Therefore, I separated the digital and analog ground plane and have on-board linear regulators instead of taking the 3.3v power from the original controller board. With this implementation, hopefully, I won’t be getting any stability issues.

I’ll update this post when I receive the board back from the PCB house, which I won’t expect to until mid January. It’s arrived!